What type of gm minivan?
I had a 1995 Lumina APV 3.8 litre and I towed my 13 foot Trillium
I barely saw a gas mileage drop.
When I got the 17 foot Trillium
, my mileage dropped to about 22mpg.
It still towed well but I added an anti-swaybar as well as air bags in my shocks.
My van was just prematurley retired at 319000 km(divide by 1.6 for miles) as the body shop it was in burned it for my Christmas present. ( I hate people who don't pay attention to what they are doing)
It still ran great, and I averaged 30mpg on the highway with 5 passengers carpooling to work.
I probably towed about 3000 miles/year with the big trailer, but never in overdrive.
I also had a brake controller, The minivan brakes
are not adequate for anything over 1500 pounds without one, and marginal for stuff over 1000 lbs. even thought they say it's ok in the book.